I've never seen Angel and Angelus as two separate entities. Angel is simply Angelus with a muzzle, that muzzle taking the form of a sense of guilt and a certain desire, within limits, to accept the responsibility for his former actions. Those limits include a determination to believe that, "I'm genuinely sorry, and I promise to try and do better in the future, but I refuse to stand trial for, or pay any penalty for, the deaths and atrocities that I have perpetrated over the last two centuries. I'll feel bad about it, and tell everyone just how bad I feel at every opportunity, and I'll help the helpless, but that's just about as far as I'll go." That for some reason makes him a hero. Spike does basically the same thing, with far less brooding and bewailing his former crimes, and he's a hypocrite and an unreformed monster who only got a soul in order to "get in a girl's pants."
Angel is Angelus with Liam's soul. Liam, though, from what little we saw of him, was a drunken, lazy, skirt chasing bum, was probably a fairly decent person, or at least not a serial killer, rapist, and sadist. With his soul restored, that personality reemerged, and, overwhelmed by what he had been taking such pleasure in doing formerly, became a recluse, alternating between periods of general normalcy and bouts of alley dwelling, rat feeding, depression. And that was the state that Whistler found him in, and that was the state in which he first saw and fell in love with a 15 or16 year old girl, cleaned his act up, and followed her to Sunnydale, where he got in her pants with disastrous results.
If you see Angelus and Angel as two separate people, then, yes, Angel is not guilty of or responsible for, anything that Angelus ever did. If that is the case, then why would he feel some overwhelming desire to atone for what his, in effect, evil twin brother had done? Obviously, he wouldn't.
Unlike Dr Bruce Banner in "The Avengers," Angel never refers to Angelus as "the other guy," or any variant thereof. He knows and accepts the fact that he IS Angelus, regardless of what name he currently answers to. He can't say "Supernatural compulsion, he wasnt Culpable, move on" because he knows that it isn't true. Whatever Angelus did IS Angel's fault.